in 4.4 there was a change in behaviour of MapReduce.
In 4.2:
"MongoDB will not call the reduce function for a key that has only a single value. The values argument is an array whose elements are the value objects that are "mapped" to the key." docs
From 4.4 onwards, the reduce function will be called regardless of how many values are associated with a given key.

Need to highlight is in the 4.4 upgrade docs, as it's braking for some customers